Nonsense-mediated decay (“NMD”) is a cellular mechanism that selectively degrades faulty messenger RNA (“mRNA”) containing an out-of-place stop (nonsense) codon. If translated, such mRNAs would produce a shortened version of the encoded protein. The NMD surveillance mechanism reduces or prevents the formation of these defective proteins and peptides. While NMD helps to protect against occasional mistakes that occur during RNA production, it also contributes to a number of genetic disorders collectively termed nonsense-associated diseases (“NADs”). Examples of NADs are cystic fibrosis and Hurler's syndrome. Depending on the gene affected and the mutation, prematurely terminated proteins may have low biological activity, no biological activity, or may even be harmful to biological functions.
In NADs there is usually an insufficient amount of the full-length protein as a result of two processes: (1) the destruction of the defective mRNA by NMD; and (2) the synthesis of truncated protein from mRNA that escapes the destruction. Currently, there are a number of drugs that are partially effective against NADs and function by suppressing nonsense codon recognition (i.e., inhibition of process (2)). NMD is triggered by a premature stop codon located at least 50-55 nucleotides upstream of a splice junction in a multi-exon mRNA. Several proteins are involved in NMD, such as Upf1, an essential protein that directly controls mRNA decay, and eIF5A, a eukaryotic initiation factor that is tightly associated with actively translated ribosomes and is an RNA-binding protein. eIF5A is the only protein known to contain the amino acid hypusine. The protein usually occurs in two isoforms, of which eIF5A-1 is usually the more abundant.
Currently, there are a number of compounds that are partially effective against NADs and function by suppressing nonsense codon recognition. While recent reports also indicate involvement of NMD in NADs and certain compounds that suppress NMD function, there is a need in the art for the further characterization of the NMD mechanism associated with various NADs, particularly with respect to inhibitors of eIF5A. There also remains a need for compounds and methods that repress NMD and thereby significantly increase the level of NMD-susceptible mRNA, thereby potentially mitigating NAD symptoms. The instant invention addresses these needs.